Various protocols are known for sharing access to a medium, e.g., polling, time division multiple access (TDMA), token passing, and carrier sense multiple access (CSMA). Polling uses a master station to periodically poll other stations (slave stations), giving the slave stations explicit permission to transmit on the medium. TDMA protocols also rely on a master station (network master), which broadcasts a frame synchronization signal before each round of messages to synchronize the clocks of all stations, and, after synchronization occurs, each station transmits during its uniquely allocated time slice. In token passing, access to the transmission medium is determined by possession of a special data unit called a token, which is passed from station to station. CSMA protocols rely on all stations listening to the medium prior to transmission to determine when the medium is idle. After the transmission ends, each station waits a specified interval (also known as an interframe gap) followed by an additional delay of one or more transmission (contention resolution) slots based on an assigned slot number before transmitting.
Carrier sense multiple access (CSMA) protocols because of their simplified access mechanism and robustness in performance under a variety of network conditions are widely used. Multimedia applications require guarantees on such parameters as latency, jitter and packet loss probability. The uncontrolled nature of channel access in CSMA networks makes it difficult to support quality of service (QoS). QoS can be evaluated in terms of latency, jitter, and packet loss probability (PLP) where latency is how much time it takes for a packet of data to get from one device to another, jitter is a random variation in the delay between the arrival of one packet of data and a consecutive packet of data, and PLP is a probability of losing a packet of data transmitted from one device to another device. Several variations to the CSMA mechanism are in use that enable various degrees of control on the QoS parameters. For example, CSMA in the HomePlug® 1.0 protocol, available from Intellon Corp of Ocala, Fla., uses a priority resolution slot mechanism. HomePlug® 1.0 protocol uses four priority levels to classify transmissions on networks. Each transmission is thus classified depending on its QoS requirements. Each class embodies a coherent set of latency, jitter and packet loss probability (PLP) guarantees. Stations having transmissions at various priority levels waiting to be sent contend during priority resolution slots of time between transmissions, causing transmissions with priority that is lower than the highest available priority in the network to defer. Thus, stations having transmissions with highest priority in the network are guaranteed access to the medium before stations having transmissions with lower priority. However, stations having transmissions with the highest priority in the network still contend within themselves for medium access, thus reducing the level of guarantees on QoS parameters. Stations at the same priority level contending for access to the medium enter a “contention window” where the stations randomly select their transmission slot time.
In some network environments, e.g., power line network environments, CSMA protocol operation may be adversely effected by distance between stations and/or channel conditions. Also, CSMA protocols may be susceptible to interference (e.g., collision) caused by overlapping networks, that is, networks that are not intended to communicate with one another but that can, in fact, hear each other's transmission, as well as by hidden stations. A hidden station is a station in a network (or overlapped network) that cannot hear all other stations because of its location and/or channel conditions. Under such conditions, a CSMA protocol may not function effectively to maintain proper network synchronization and provide orderly media access arbitration. And a CSMA protocol may be unable to ensure strict adherence to QoS guarantees. For example, U.S. application Ser. No. 09/632,609 describes a system in which a single contention-free interval can be established on a CSMA network without the use of a master, but this method may be insufficient to handle strict quality of service (QoS) requirements or hidden nodes.